Due to dwindling supplies coupled with increasing demand, the price of oil has, and will continue to increase substantially over the years. The increasing price of oil, along with an increased scrutiny on the effects of greenhouse gas emissions, has led to the evaluation of alternative fuel sources to meet the energy demands and address environmental concerns. One such alternative fuel is the production of crude oil and biodiesel from vegetative precursors, such as algae.
A principal component of algae's composition is lipid oil which can be converted into a crude-type oil, consisting primarily of single-chain hydrocarbons or triglyceride and di-glyceride fats and oils, for biodiesel feedstock. Algae has the benefit of being able to be grown in massive quantities with very little environmental impact. All that is needed to grow algae is water, appropriate nutrients, sunlight and carbon dioxide. Thus, as compared to petroleum, oil and biodiesel produced from algae are not a limited resource, because algae can be continuously grown in mass quantities for fuel production. Moreover, as compared to food crop biodiesel and ethanol produced from feed crops (i.e., grains), the production of algae does not drive up the price of certain food products and has a higher level of efficiency. For example, soy or corn yields approximately 70-100 and 150-300 gallons of fuel per acre per year, respectively. In contrast, certain algae species can yield in excess of 10,000 gallons of fuel per acre per year.
In addition, algae can provide several other benefits. For example, algae can yield specialty chemicals and/or pharmaceuticals (i.e., plastic resins (such as PHA and PHB), ketones, acetone, beta-carotene and Omega-3 and the like), nutrients, and a food source for animals, fish and humans. The challenge for producers of algae is not only to identify the most efficient strains of algae to use for the desired end-product, but also to determine how algae best can be grown to meet the demand for such end-products.
The most natural system for growing algae is the open-pond system (e.g., raceway ponds or natural ponds). Open-pond systems allow for algae growth in its natural environment and minimize environmental impact. While an open-pond system offers a low-cost algae production environment with very little environmental impact, open-pond systems inherently present too many variables to be controlled for maximized algae production. For example, open-pond systems are more susceptible to contamination from bacteria or other organisms that can stunt algae growth and make it difficult to target desired species of algae. Further, algae need to be shielded from bad weather and the water needs to be adequately stirred to promote algae growth, which is difficult and expensive to control in open-pond systems. As a result of all of these variables, open-pond systems suffer from low and/or inconsistent productivity levels.
In attempts to maximize yield and increase the speed of algae production, algae producers have utilized photoautotrophic and heterotrophic methods of algae production. Photoautotrophic methods utilize light to produce biomass, while heterotrophic methods involve algae consumption of sugars to produce biomass. Photoautotrophic algae producers use closed-loop systems, such as bioreactors or closed tank systems. Bioreactors involve the use of an array of vessels, typically bags or tubes, filled with an algae culture and media to maximize sun exposure and algae production. Closed tank systems involve the use of round drums and a controlled environment to maximize algae production. Heterotrophic systems, such as fermentation systems, are also being tested and developed in attempts to maximize the production of algae. The problems with all of these systems to date is that they each suffer from extremely high production costs that are so cost prohibitive that only small scale uses of these systems are economically feasible.
For photoautotrophic algae production methods, the focus is on optimizing photosynthesis to promote algae growth. Plants derive energy from sunlight and use that energy to convert carbon dioxide and water into biomass. Uncultivated macroscopic green plants have an energy utilization efficiency of approximately 0.2% (i.e., 0.2% of incident sunlight is utilized by the plant to convert water and carbon dioxide into biomass). Plants species can be classified by referring to their carbon fixation process (e.g., C3-cycle plant species and C4-cycle plant species), which is the first step of converting sunlight to biomass in photosynthetic organisms. Plant cultivation can improve energy utilization to a range of 1-2% for C3-cycle plant species and up to about 8% for the most productive C4-cycle plant species (e.g., sugarcane). Uncultivated microscopic green algae (typically C3-cycle plants) are more efficient than macroscopic plant species and can average as much as 6.2% energy utilization efficiency. Thus, by cultivating algae in controlled environments, the energy utilization efficiency can be increased even more and the rates for growing algae can substantially be increased.
Algae grows best at low light levels because at low light levels, algae photoefficiency can be as high as 60% to 80%. Counter-intuitively, high light levels decrease production, because algae respond to high light levels by protecting themselves from excessive radiation through the mechanisms of photoinhibition and photorespiration. Photoinhibition is the production of light absorbing materials to protect the algae's light harvesting chlorophyll antennas from damage caused by light over-saturation. Photorespiration essentially short-circuits the photosynthesis process because of excess production of oxygen. The result is that oxygen out-competes carbon dioxide at the site of the Rubisco enzyme and glucose cannot be produced. Thus, to keep algae biomass production occurring at a high rate, the light levels must be low enough so that carbon fixation does not exceed the concentration dependent diffusion rates of carbon dioxide into the algae's chloroplasts.
It also needs to be kept in mind that photosynthesis does not use a large proportion of the sun's broad light production. Even though the sun has its highest output in the green portion of the spectrum (around 550 nm), algae only use the light in portions of the red and blue regions of the spectrum. The inactive portions of the spectrum, such as ultraviolet and infrared portions, contain quite a bit of energy which constitutes a large fraction of the solar output. Unfortunately, these inactive portions often cause more harm than good in the algae growing process because ultraviolet radiation can cause damage and resulting oxidative stress. Infrared radiation can also cause significant and potentially damaging over-heating of the algae.
To prevent the problems associated with over radiation, algae producers can use some means of shifting the sun's illumination to match the photosynthetic action spectra. Such tools can involve the use of light sources, such as highly efficient blue and red LEDs, that effectively and efficiently produce photosynthetically active radiation (PAR). However, the use of such light sources have the negative impact of increasing the cost of production because they increase the amount of energy needed to power the production process.
Thus, a photobioreactor system and method for producing algae is still needed that optimizes the available sunlight and maximizes the production of algae in a low-cost, efficient manner in order to make large scale algae production economically feasible. The present novel technology addresses this need.